The present invention relates to thin film fluid control systems and, more particularly, to the fabrication of microelectromechanical shape-memory thin film fluid control systems such as peristaltic pumps, for example, and methods of fabricating such devices.
The shape-memory effect employed under the present invention arises in certain metal alloys that undergo reversible displacive transformations (`martensite` transformations) in which, on cooling, the crystal lattice shears to a new crystal symmetry, with no diffusion occurring, and very little volume change. The strain produced during a shape-memory transformation can produce very large forces that are useful to carry out work.
To produce a shape-memory displacement, the appropriate alloy is first cooled to a temperature at which the structure is fully martensitic, that is, to a temperature below the `martensite-finish` temperature, hereinafter M.sub.f. If the cooling is done in the absence of stress, the martensite is self-accommodated, and contains a nearly random distribution of 24 variants of the sheared structure, such that the macroscopic deformation is nominally zero. If the martensite is now subjected to an applied stress it can be permanently deformed, but the deformation which takes place is not mediated by dislocation slip, but rather by a cooperative rearrangement of the individual martensite variants via twin-boundary motion. Shape-change occurs by motion of the intervariant boundaries in a way which increases the size of the shear-variants aligned with the deforming stress, while variants which have shears in the antisense of the applied stress shrink or disappear.
The shape-memory effect is manifest when this material is heated to a temperature at which the martensite reverts to the high-temperature phase. This process begins at A.sub.s, the austenite-start temperature, and is complete at A.sub.f, the austenite finish temperature. During the transformation, each variant transforms from its monoclinic (low-symmetry) structure back to the high temperature (cubic) structure; since the crystal symmetry is increased, each martensite variant has only one way it can shear to form the parent symmetry. This shear is always the exact opposite of the shear which initially formed the variant, and the material therefore recovers not only the transformational shears (which were self-accommodated) but also the shears imparted by the deforming stress, since the latter were `descended` from the original variants. The result is a recovery (`memory`) of the shape held prior the deformation of the martensite.
If the alloy is again cooled below M.sub.f, the transformation once again produces a `random` martensite that will not exhibit shape-memory unless it is again deformed by application of external stress. Thus the exploitation of the effect to make a reversible, cyclic actuator requires the use of a `biasing` element to provide the force needed to deform the martensite phase on cooling.
The so-called shape memory effect is described in relative detail in U.S. Pat. No. 5,325,880 by Johnson et al. which describes a microvalve in the form of a poppet formed of a shape memory alloy. The poppet is suspended within a pressure cavity defined by a base and a valve cap formed of silicon. By applying heat to the shape memory alloy, the alloy blocks the port to preclude fluid flow.
U.S. Pat. No. 5,619,177 which is also by Johnson et al. relates to a more elaborate microactuator wherein the actuator member is comprised of a shape memory alloy layer, an elastic substrate, a base including a first layer of silicon and a second layer of a charge carrying material. In order to activate the apparatus, a heating circuit and a clamping circuit are provided whereby one is held open and the other is held closed to open or close an orifice provided within the base.
While the devices described in the foregoing patents may be useful for the stated purposes, the apparatuses disclosed do not appear to be useful in association with tubular conduits generally and, more particularly, delicate conduits such as biological conduits, i.e., blood vessels and urethras by way of non-limiting example. Neither of the Johnson et al. references appear to be capable of a retrofit application, i.e., installed and used in association with an existing conduit. As such, there is a clear need for the thin film fluid control systems of the present invention.